In the field of thermal printing, it is well-known that a significant limitation in the printing operation is the absence of copies during such printing operation. The early thermal printing systems used a thermal print element energized to heat specific and precise areas of a heat-sensitive paper or like record material and thereby produce readable characters on the single sheet of paper. In this respect, the single sheet of paper includes material which is reactive to the applied heat and is described as a self-contained system.
More recently, thermal printing systems have included two separate sheets of paper or like record material, wherein each sheet is coated with a heat-sensitive reactive material. The top or front sheet is usually a light weight tissue-type paper which is coated with the heat-sensitive material and the second sheet is preferably bond-type paper which is also coated with the sensitive material. The two sheets are then mated or collated in a manner wherein the uncoated side of the tissue paper is in contact with the coating on the bond paper. The coated side of the tissue paper is adjacent and in close proximity to the thermal printing elements, or in certain applications, the elements may be in actual contact with the tissue paper. The thermal elements are actuated to provide specific and precise marking or imaging on the papers in the process which enables the obtaining of a master sheet plus a readable copy.
Alternatively, the tissue paper could be coated on both the front and back sides so that the thermal printing elements are adjacent or in contact with the front coated side of the tissue paper and the back coating is in contact with the bond paper. In similar manner, the thermal elements are actuated to provide the specific and precise marking or imaging on the two papers so as to be in readable form. The tissue sheet and the bond sheet are arranged in manifold manner and the imaging is accomplished by transfer of the ink or like material in the coating onto the sheet.
Representative documentation in the field of thermal printing includes U.S. Pat. No. 3,539,375, issued to H. H. Baum on Nov. 10, 1970, which discloses temperature-responsive record material that includes a support sheet having crystal violet lactone and a phenolic disposed in a matrix of polyvinyl alcohol, and arranged such that application of heat will produce a mark-forming reaction between the lactone and the phenolic.
U.S. Pat. No. 3,561,991, issued to H. H. Baum on Feb. 9, 1971, discloses a transfer record sheet for making multiple copies of a single heat impression wherein the translucent support sheet is coated with an ink source that normally is solid at room temperature and meltable to a tacky transfer condition upon application of heat. The melted coating stays in the tacky condition for a period of time to allow the making of a succession of copies upon contact of sheets with the melted image area.
U.S. Pat. No. 3,674,535, issued to J. H. Blose et al. on July 4, 1972, discloses heat-sensitive record material comprising a paper base sheet and a coating of chromogenic material and a bisphenol distributed in a polyvinyl alcohol in combination with a filler, a lubricant and a non-tacky wax.